Laser powder bed fusion of Ti-6Al-4V parts: Thermal modeling and mechanical implications

Abstract

A continuum-scale modeling approach is developed and employed with three-dimensional finite element analysis (FEA), for simulating the temperature response of a Ti-6Al-4V, two-layered parallelepiped with dimensions of 10×5×0.06mm3 during Laser Powder Bed Fusion (L-PBF), a metals additive manufacturing (AM) method. The model has been validated using experimental melt pool measurements from the literature and also accounts for latent heat of fusion and effective, temperature-dependent transport properties. The discretized temperature, temperature time rate of change (i.e. cooling rate) and temperature gradient are investigated for various scan strategies and number of lasers, i.e. 1, 2 or 4. The thermal response inherent to multi-laser PBF (ML-PBF) is investigated. The number of sub-regional areas of the powder bed dedicated to individual lasers, or ‘islands’, was varied. The average, maximum cooling rate and temperature gradient per layer, as well as the spatial standard deviation, or uniformity, of such metrics, are presented and their implications on microstructure characteristics and mechanical traits of Ti-6Al-4V are discussed. Results demonstrate that increasing the number of lasers will reduce production times, as well as local cooling rates and residual stress magnitudes; however, the anisotropy of the residual stress field and microstructure may increase based on the scan strategy employed. In general, scan strategies that employ reduced track lengths oriented parallel to the part's shortest edge, with islands ‘stacked’ in a unit-row, proved to be most beneficial for L-PBF.